A Vocabulary preview 1 Complete the text using the words in the box. Change the form if necessary. absorb advance air pressure determine echo reflect sound wave vibration Six hundred years BCE, the Greek scientist Pythagoras proposed the principle that sound is caused by^1 __ in the air.Essentially, these are moving and changing the^2 __ around the sound. Not long after this, the process of sound traveling through the air to the ear drum was discovered.By the 19^th century, our understanding had 3 __ significantly Research was now focusing heavily on the idea of 4 __ This showed the frequency at which different sounds traveled.The number of waves per second5 __ the frequency. Researchers also started to understand the importance of the environment on the sound created.Some materials would6 __ the sound whereas others would^7 __ the sound and rebound it back to the source, creating a strong^8 __